Single-tube high-wattage short-arc metal halide lamps with 2000 W-3000 W power rating, which are one class of high-intensity-discharge (HID) lamps, can be suitably used for compact fixtures, because they are compact lamps due to their single-tube structure. They are especially advantageous for use in lighting facilities of a sports stadium or the like, where they can contribute to the reduction of construction costs. Light control is easy because of the short-arc characteristics and it is possible to keep the amount of light leaking from the stadium area to a minimum and use an illumination design that prevents light pollution. Therefore, single-tube high-wattage short-arc metal halide lamps are widely used for out-door illumination of sports grounds.
However, since such metal halide lamps do not have an outer tube, there is the danger that fragments of the discharge vessel may scatter into the inside of the appliance or fixture when the discharge vessel is damaged for some reason while the lamp is turned on, collide with the appliance or fixture and damage its shield glass.
In order to reduce the shock of such an impact, usually the mercury density in the discharge vessel of the burning single-tube high-wattage short-arc metal halide lamp is reduced to decrease the internal pressure of the discharge vessel. Since reduction of the mercury density also lowers the lamp voltage, lamps with a lamp voltage of 100V rated tube voltage are widely used.
In conventional single-tube high-wattage short-arc metal halide lamps with 100V rated tube voltage however, the tube current in the case of 2000 W tube power becomes about 20 A and tube current in the case of 3000 W tube power becomes about 30 A. Thus, the current becomes quite large.
A single-tube high-wattage short-arc metal halide lamp has a pair of electrodes in the discharge vessel, and lamp shafts into which a feedthrough has been sealed are connected to both sides of this discharge vessel. The feedthroughs comprise a conductive foil of molybdenum connected to a lead rod. In the feedthroughs, heat is released at the conductive foil and the lead rod as well as at the junction of the conductive foil and lead rod, because at 2000 W and 3000 W power dissipation the tube current is rather high with 20 A or 30 A respectively. The feedthroughs can deteriorate by oxidation due to this heat release, so that the life expectancy of the lamp is shortened.
In order to overcome these problems of the prior art, it is a purpose of the present invention to provide a metal halide lamp for a rated voltage of 200V that has a low internal pressure in the discharge vessel when the lamp is burning and does not harm the appliance or fixture even when the discharge vessel is damaged while the lamp is burning.